Many foods are cooked in the presence of added water or moisture generated by components of the food itself. In conventional cooking, such foods may simply be placed in a container with a cover that is capable of restricting or eliminating the loss of moisture from the food as it is cooked. Other foods may be completely submersed in water for the duration of the required cooking time, after which the water is removed and the food served.
For certain foods, specially designed cooking containers known as "steamers" cook food by heating water to produce steam that is then brought into contact with the food to achieve the cooking process. Long before microwave cooking was developed, cooking with steam was a technique that was widely practiced. Steam cooking offers several advantages, aesthetic and otherwise, in preparing certain foods. For many foods, vegetables in particular, steam cooking is a favored cooking method for preserving the food's texture, color, taste, and nutritional value. The steam is produced by heating a quantity of water to or near its boiling point and allowing the steam to circulate within an enclosed space.
Foods cooked in steam tend to retain water-soluble vitamins and minerals that may be lost if the food were boiled in a quantity of water that is discarded after cooking. Moreover, certain steam-cooked foods retain their color and texture, making the food more aesthetically pleasing than the same food cooked by a different method. Additionally, when food is cooked with steam, the potential for burning the food is reduced because the steam moderates the heat, and the moisture prevents food from becoming dried.
Microwave cooking, since its inception, has become increasingly popular because of economy of time, effort, and energy. However, because microwave ovens cook with transmitted energy rather than through conduction or convection of heat energy, certain problems unique to microwave cooking have arisen. For example, certain substances accumulate heat and cook faster than others, leading to hot and cold spots in food. Microwave ovens are notorious for not distributing microwave energy in a uniform or predictable fashion. Also, certain foods are not aesthetically pleasing when cooked in a microwave oven for a variety of reasons. Additionally, loss of moisture during the cooking process causes many foods to obtain an unpleasant texture.
Despite the known problems, microwave ovens are widely used for reheating, and to a lesser extent, for cooking of many foods. Through trial and error most people develop preferred methods for cooking particular foods based on the aesthetic considerations associated with particular foods and individual tastes. For a few foods, special cooking containers have been developed. Some of these containers attempt to mimic the effect of a grill or griddle, others, such as microwave-compatible bags of popcorn, are simply containers that are conveniently used with certain types of food as dictated by the physical characteristics of the food and the method of cooking that yields desirable aesthetic effects for a particular food.
Although special microwave cooking containers may achieve desirable results for a particular food, the design of many of these containers is suitable for use only with those foods. For example, U.S. Pat. No. 4,280,032 describes an apparatus having an outer container containing a water chamber in which is placed an inner container, preferably composed of microwave-energy impermeable drawn aluminum, containing an egg. This design is chosen to address certain problems inherent in cooking eggs because an egg has unique physical properties that are problematic when microwave energy is used for cooking.
Another example of a container specially designed for the characteristics of a single food is U.S. Pat. No. 4,853,509. This container is specially designed to allow a user to boil and steam rice in a desired shape without the necessity of handling a quantity of the hot rice to press it into a desired shape.
With the advent of microwave cooking, a number of containers have been developed that attempt to overcome the problem of uneven heating that occurs in conventional microwave ovens. Uneven cooking, resulting in hot and cold spots, is a problem experienced by most, if not all, persons who cook with a microwave oven. Attempts to combat uneven heating have involved rotating the food, either manually, which requires interrupting the cooking cycle, or on a rotating platter installed in the microwave oven.
In the attempt to overcome the uneven cooking problem, some approaches recognize the difficulty in heating a large single mass of food and provide a container that exposes the food to a more uniform flux of microwave energy. For example, U.S. Pat. No. 4,416,906 describes a disposable microwave shipping and cooking package that packs the food around a central core composed of microwave transparent material in an attempt to encourage complete heating of the innermost parts of the food mass. The physical configuration of the package and the orientation of the pre-packaged food appears to offer the advantage of allowing microwave penetration and heating of the food from the direction of the inner core. Microwave energy does not penetrate the sides of the container, and the heating of the food appears to begin at the innermost portion of the food mass and proceeds outward.
Many books that instruct in microwave cooking techniques advise that vegetables and such foods that may freely contact water should be cooked in a shallow container with the food partially submerged in water. This approach is an attempt to preserve the moisture content of the food, especially on the foods exterior surface and to use the liquid medium to promote more uniform heating. However, this method is similar to traditional boiling and adversely effects the aesthetic qualities, especially texture and to a lesser extent, color, of many foods and may cause loss of certain water-soluble vitamins. As an alternate approach, containers have been developed that allow the food to rest on ridges or other supports above the level of water added to the container. Moisture rising from beneath the food helps prevent excessive moisture loss from the food while the cooking is achieved by the microwave energy.
Another approach used in the design of microwave cooking containers is the use of materials that partially or totally shield the enclosed food from microwave energy. It is believed that these designs reflect attempts to mimic traditional cooking methods in microwave ovens. U.S. Pat. No. 4,317,017 describes an adapted steaming apparatus that shields food from microwave energy but features a microwave transparent water containment chamber beneath the food containment chamber. Water is placed in the dish portion and, as heating by microwave energy occurs, steam rises through openings in the bottom of the food containment chamber. In this apparatus, heating of the food is caused solely by steam rising into the food containment chamber. Although this design appears to create and contain steam, certain foods are unsuitable for cooking by steam alone, and this device is not designed to allow penetration of microwave energy. Moreover, there is no means to encourage thorough circulation of the steam. A design of this type requires that the food rest above a reservoir of water and necessitates use of an internal container, grate, or shelf to keep the food out of the water reservoir.
U.S. Pat. No. 4,705,927 describes a device where water is placed in a microwave permeable container beneath the food containment chamber. The steam generated heats the bottom of the food containment chamber which in turn conducts heat to the food as in conventional cooking. Additional cooking is achieved by exposure to microwave radiation through the top of the container. Steam is not circulated in the food-containing compartment used to promote more thorough heating via heat conducted to the food containment chamber by steam created in the water chamber.